fix negated error codes from ptsname_r

the incorrect error codes also made their way into errno when
__ptsname_r was called by plain ptsname, which reports errors via
errno rather than a return value.

fix ptsname_r to conform to the upcoming posix requirements

it should return the error code rather than 0/-1 and setting errno.

add ptsname_r (nonstandard) and split ptsname (standard) to separate file

this eliminates the ugly static buffer in programs that use ptsname_r.

initial check-in, version 0.5.0