The MySQL implementation of strmov() supports overlapping
source and destination. This makes sense given the name
as it is similar to the C API function memmove().

However, on platforms which support the POSIX function stpcpy(),
we did "#define strmov(A,B) stpcpy((A),(B))". This is a problem
as stpcpy() does not support overlapping source and destination.
Therefore strmov() could have different behavior from platform
to platform.

Another (smaller) issue, was that strmov() sounds like it
returns a pointer to the destination (like strcpy()) and not,
as in fact does, a pointer to the terminating NULL (as stpcpy()).

This patch does the following:
1) Renames strmov() to my_stpmov() and strnmov() to my_stpnmov().
2) Adds my_stpcpy() which is stpcpy() if supported,
   my_stpmov() otherwise.
3) Adds my_stpncpy() which is stpncpy() if supported,
   my_stpnmov() otherwise.
4) Replaces existing calls to strmov() with my_stpcpy().
5) Replaces existing calls to strnmov() with my_stpncpy() or
   my_stpnmov().
5) Removes strmov_overlapp() and replaces its usage by my_stpmov().