Clobber strings with 0xf1f1f1f1 before writing to them to check buffer

Clobber strings with 0xf1f1f1f1 before writing to them to check buffer
lengths are correct.  Attempts to pstrcpy into an fstring or allocated
string should fail in developer builds.

This builds on abartlet's earlier overflow probe for safe_strcpy, but
by clobbering the whole string with a nonzero value is more likely to
find overflows on the stack.

This is only used in -DDEVELOPER mode.

Reviewed by abartlet, tpot.
(This used to be commit 8d915e266cd8ccc8b27e9c7ea8e9d003d05f8182)