add missing confstr constants

the _CS_V6_ENV and _CS_V7_ENV constants are required to be available for use
with confstr. glibc defines these constants with values 1148 and 1149,
respectively.

the only missing (and required) confstr constants are
_CS_POSIX_V7_THREADS_CFLAGS and _CS_POSIX_V7_THREADS_LDFLAGS which remain
unavailable in glibc.

fix confstr return value

per the specification, the terminating null byte is counted.

put confstr.c with the other conf functions