r23518: Remove the silly assumption that string_replace requires a pstring.

r23518: Remove the silly assumption that string_replace requires a pstring.

Jeremy, I am always very confused about the different length arguments
in convert_string and friends. Can you take a look at the change in
string_replace and verify it's ok? Thanks!

While at it, remove the pstring limit for strhasupper and strhaslower.
(This used to be commit e6e5703658aeaeb0cca5d7281095e17553001d4b)