gdbsrv: factorize gdb version handling, fix doc and typos

gdbsrv: factorize gdb version handling, fix doc and typos

* fix various typos in doc
* following commit in gdb
   http://sourceware.org/ml/gdb-patches/2011-07/msg00742.html
  means unlimited length for valgrind watchpoints is understood by the
  (future) gdb 7.4 => doc updated
* factorize  gdb version detection and reporting in
  gdbserver_tests/make_local_links
* replace zignal by signal in a string used in umsg.
* updated gdbserver_tests/README_DEVELOPPERS (ref to --port vgdb option)

No functional change, tested on f12/x86, debian5/amd64, s390/RHEL4

Fixes #278892.  (Philippe Waroquiers, philippe.waroquiers@skynet.be)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12223 a5019735-40e9-0310-863c-91ae7b9d1cf9