Initialization of sensors module fixed

- Debian doesn't contain file "/etc/sensors.conf", there is "/etc/sensors3.conf" instead, so the module initialization will always fail.
- It is possible to pass NULL to sensors_init(). The library will find the correct file by itself in such case. This approach is also suggested in manual, so it was choosen as a fix.
- sensors_init() manual page:

If FILE is NULL, the default configuration files are used (see the FILES section below). Most applications will want to do that.

FILES
       /etc/sensors3.conf
       /etc/sensors.conf
              The system-wide libsensors(3) configuration file. /etc/sensors3.conf is tried first, and if it doesn't exist, /etc/sensors.conf
              is used instead.

       /etc/sensors.d
              A directory where you can put additional libsensors configuration files.  Files found in this directory will  be  processed  in
              alphabetical order after the default configuration file. Files with names that start with a dot are ignored.
Compiler warning enabled

- "-Wall -Wextra" added to CFLAGS to enable reporting of all compiler warnings.
- "doc" and "build" directories added to .gitignore.
Compiler errors fixed in sensors module

- sensors_feature_data structure was renamed to sensors_feature.
- sensors_get_all_features() function was replaced by sensors_get_features() and sensors_get_all_subfeatures(). (The use of the second one will be implemented in the next commits.)
- sensors_get_feature() was renamed to sensors_get_value().
- Parameters and return value of sensors_get_label() were changed.
- Testing: the code returns data from the sensors

xsysguard/src/xsysguard -H sensors
sensors - libsensors (lm-sensors - Linux hardware monitoring)

SENSORS_CHIP0:   acpitz
SENSORS_CHIP1:   coretemp
SENSORS_CHIP2:   thinkpad

N sensors:acpitz:temp1                             35.00
N sensors:acpitz:temp2                            127.00
N sensors:coretemp:temp2                             34.00  Core 0
N sensors:coretemp:temp3                            100.00  Core 1
N sensors:thinkpad:fan1                             1961.00
N sensors:thinkpad:temp1                             35.00
N sensors:thinkpad:temp2                             37.00
N sensors:thinkpad:temp3                             32.00
N sensors:thinkpad:temp9                             37.00
N sensors:thinkpad:temp10                            44.00
N sensors:thinkpad:temp11                            36.00
sensors_get_all_subfeatures() is used in sensors module

- Data returned from sensors_get_all_subfeatures() are used the same as the one from parent sensors_get_features().
- sensors_get_label() expects only feature_data structure, so it is not used for subfeature_data. (Is the libsensors API complete? The compiler warning is pretty clear here.)
- The "number" parameter of sensors_get_value() is declared as number of subfeature, but number of feature can be used here too. (I hope that I understand the libsensors API correctly. sensors_feature structure is commented with "Data about a single chip feature (or category leader).")
- SENSORS_NO_MAPPING constant was removed from libsensors in the past, so the codes commented out using "#if 0" were removed too - it will be never enabled.
- Testing: the code returns data from the sensors

xsysguard/src/xsysguard -H sensors
sensors - libsensors (lm-sensors - Linux hardware monitoring)

SENSORS_CHIP0:   acpitz
SENSORS_CHIP1:   coretemp
SENSORS_CHIP2:   thinkpad

N sensors:acpitz:temp1_input                       35.00
N sensors:acpitz:temp1_crit                       127.00
N sensors:acpitz:temp1                             35.00
N sensors:acpitz:temp2_crit                       100.00
N sensors:acpitz:temp2                            127.00
N sensors:coretemp:temp2_input                       33.00
N sensors:coretemp:temp2_max                        100.00
N sensors:coretemp:temp2_crit                       100.00
N sensors:coretemp:temp2_crit_alarm                   0.00
N sensors:coretemp:temp2                             33.00  Core 0
N sensors:coretemp:temp3_max                        100.00
N sensors:coretemp:temp3_crit                       100.00
N sensors:coretemp:temp3_crit_alarm                   0.00
N sensors:coretemp:temp3                            100.00  Core 1
N sensors:thinkpad:fan1_input                       1928.00
N sensors:thinkpad:fan1                             1928.00
N sensors:thinkpad:temp1                             35.00
N sensors:thinkpad:temp2                             37.00
N sensors:thinkpad:temp3                             32.00
N sensors:thinkpad:temp9                             36.00
N sensors:thinkpad:temp10                            44.00
N sensors:thinkpad:temp11                            36.00
Compiler warnings -Wall enabled and solved

- Argument -Wall is passed to compiler in all Makefiles.
- Return value of XQueryTree() was stored but it was never tested. Zero is returned on error (http://tronche.com/gui/x/xlib/introduction/errors.html#Status).