psql crash fix

I found that psql will crash if given a PSQLRC value containing a tilde:

    $ PSQLRC="~/x" psql test
    *** glibc detected *** psql: free(): invalid pointer: 0x00007fffb7c933ec ***

This is on Debian Squeeze 6.0.7.  The fix is to pstrdup() the value
returned by getenv(), so it can be free()'ed later --- you can't free
getenv()-returned values:

       As typically implemented, getenv() returns a pointer to a string
       within the environment list.  The caller must take care not to
       modify this string, since that would change the environment of
       the process.

This bug exists in 9.2 and git head.  I also removed the return value
from expand_tilde() as no caller was using it.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +