saleae-logic16: Don't send more samples than requested.

This fixes bug #350.
saleae-logic16: Use unitsize 1 if none of channels 8-15 are used.
saleae-logic16: Minor whitespace fixes, cosmetics.
asix-sigma: Publish capture ratio correctly.
asix-sigma: Disable sample-limited capture for now.

This is not really supported yet in the driver.
Don't define names ending with _t (POSIX reserved).

Avoid defining any names ending in _t, those are generally reserved
for POSIX usage. For details see:

http://pubs.opengroup.org/onlinepubs/007904975/functions/xsh_chap02_02.html
http://www.gnu.org/software/libc/manual/html_node/Reserved-Names.html
Drop left-overs of the removed alsa driver.
README.devices: Various updates and fixes.
Drop obsolete Linux-only usbtmc backend.

We're now using a portable libusb-based (userspace) backend which should
in theory work on any OS with libusb support.
Unhook the Link Instruments MSO-19 from the build for now.

This driver is neither working nor has it been in a compiling state for
a long time, so unhook it from the build until it is fixed and works.

The files (api.c and protocol.[ch]) are still in git, but won't end up in
released tarballs and they don't get built (neither git nor tarballs).

This also allows us to drop the otherwise unneeded dependency on libudev.
When the MSO-19 driver comes back, it should be in a form that doesn't
require the inherently Linux-only libudev anyway. See also:
http://sigrok.org/bugzilla/show_bug.cgi?id=65
doxygen: Updated Doxyfile to doxygen 1.8.6.
asix-sigma: Don't set invalid configuration options.
libsigrok.h: Add some missing enum names for consistency.
Add/update Doxygen @since tags.
log.c: Avoid using the same name for a variable and enum.
README: Mention missing dependency 'librevisa'.
rigol-ds: Quick hack to fix bug #354.
Rename SR_T_CHAR to SR_T_STRING.

This is more correct and less confusing.
NEWS: Add list of user-visible changes so far.
configure.ac: Bump libtool/library version from 1:2:0 to 2:0:0.

The libtool current:revision:age numbers change from 1:2:0 to 2:0:0.

Details:
http://www.gnu.org/software/libtool/manual/libtool.html#Updating-version-info

This changes the library filename (e.g. on Linux) from libsigrok.so.1.0.2
to libsigrok.so.2.0.0, the SONAME (+symlink) becomes libsigrok.so.2.