prevent rounding 0 if HWTIMER_SPEED > 1000000L

The current macros in hwtimer.h expect HWTIMER_SPEED to be < 1000000L, otherwise integer arithmetic will round the result down to 0.
Add a case to prevent that.
tests: extend cib unittest
core: inline `cib.c`

The functions in `cib.h` are only used in `msg.c`. They are very small,
and proper function calls have a bigger overhead than inlining these
functions.

The difference in .text size (worst to best):

```
+104 native
+69  qemu-i386
+32  mbed_lpc1768
+28  samr21-xpro

-54  chronos
-54  z1
-56  iot-lab_M3
-56  msb-430
-56  msb-430h
-56  msba2
-56  openmote
-56  spark-core
-56  telosb
-56  wsn430-v1_3b
-56  wsn430-v1_4
-58  arduino-mega2560
-60  airfy-beacon
-60  arduino-due
-60  f4vi1
-60  msbiot
-60  pca10000
-60  pca10005
-60  stm32f0discovery
-60  stm32f3discovery
-60  stm32f4discovery
-60  udoo
-60  yunjia-nrf51822
-72  avsextrem
-72  pttu
-120 redbee-econotag
```
C++ -_-
core: cib size must not exceed MAXINT/2
doc: added missing closing doxygen brackets
doc: use @brief for single defines
doc: added missing titles
doc: fixed typos and other misspellings
doc: use @false instead of 0
doc: use doxygen li tags
doc: add to group instead of redefining
doc: fix typo in UDP header
doc: remove doxygen duplication
doc: removed broken doxygen struct tags
doc: removed superfluous (and broken) @file params
doc: fixed broken param descriptions
net: socket: fix for e93d030058684f98894aafb5c4048b2c6d9d479f

The former fix for socket initialization was broken. This fixes the
"fix" by using the right exit condition for the loops.
socket: cosmetics
socket: socket_base_exists_socket returns bool

Adapt the prototype to the actual used return value.