fix compile errors with gcc 4.1.x and 4.2.x

- tested with gcc 4.1.2, 4.2.1, 4.4.6, 4.6.3 and 4.7.2
- disable gcc ignore pragma for `-Warray-bounds` if gcc version < 4.3
   - when I took out the pragma tvheadend compiled fine in all the above compilers, but I assume some version of gcc is issuing a false-positive.
   - "-Warray-bounds" was [added in gcc 4.3](http://gcc.gnu.org/gcc-4.3/changes.html): 
- In main.c, `*r` is initialized to null due to a "uninitialized variable" warning in gcc when strtok_r is inlined.  See [gcc bug 26634](http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26634).
- fixes typo in cmdline help for -C
- clarify usage of -a in help description

Here's the stderr output from gcc4.2.1 prior to this fix:
```
cc1: warnings being treated as errors
/home/acm/dev/tvheadend/src/main.c: In function ‘main’:
/home/acm/dev/tvheadend/src/main.c:479: warning: passing argument 1 of ‘__strtok_r_1c’ discards qualifiers from pointer target type
make: *** [build.linux/src/main.o] Error 1
cc1: warnings being treated as errors
/home/acm/dev/tvheadend/src/capmt.c:78: warning: unknown option after ‘#pragma GCC diagnostic’ kind
make: *** [build.linux/src/capmt.o] Error 1
make: Target `all' not remade because of errors.
```