lib/, po/, src/: Rename stpeprintf() => seprintf()

lib/, po/, src/: Rename stpeprintf() => seprintf()

The old name was too complex, and is inconsistent with all other
sprintf(3)-based APIs having just one letter for differentiation.
This allows breaking less lines.

The original name was chosen for differentiation with the buggy Plan9
API seprint(2).  However, 9front (the current fork where Plan9 is mainly
developed) has acknowledged the bug.  There's still no decision on
fixing the bug or not, due to the age of their code base, and the
projects depending on their library.  It is under consideration
inventing something like a seprint2(2) in 9front for replacement of
seprint(2), but there's no decision yet either.

Considering that 9front acknowledges their bug, and that they *may*
release a fixed API with a similar name, we may as well claim that our
seprintf() is also a fixed version of Plan9's seprint(2).  It has a
different name, after all (we terminate in 'f').

This commit was partially scripted with

	$ find * -type f \
	| xargs grep -l stpeprintf \
	| xargs sed -i 's/stpeprintf/seprintf/g';

Signed-off-by: Alejandro Colomar <alx@kernel.org>