Fix fwrite() reading beyond end of buffer in error path

Partially revert commits 2b766585f9b4ffabeef2f36200c275976b93f2c7 and
de2fd463b1c0310d75084b6d774fb974075a4ad9, which were intended to fix BZ#11741
but caused another, likely worse bug, namely that fwrite() and fputs() could,
in an error path, read data beyond the end of the specified buffer, and
potentially even write this data to the file.

Fix BZ#11741 properly by checking the return value from _IO_padn() in
stdio-common/vfprintf.c.
Add systemtap markers to math function slow paths

Add systemtap probes to various slow paths in libm so that application
developers may use systemtap to find out if their applications are
hitting these slow paths.  We have added probes for pow, exp, log,
tan, atan and atan2.