openssh: minor fixes for --with-keychain-support

Made --with-keychain-support option visible, and fixed caveats to use
the correct Homebrew prefix.

Closes #224.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
gdb: add version suffix

* Using "--with-version-suffix", gdb executable will be named as "gdb-7"
  where "7" is the major version.  This makes using system gdb and
  Homebrew gdb at the same time easier.
gdb 7.6.1