Auto merge of #1494 - lucab:ups/linux-gnu-utmpname, r=gnzlbg

linux/gnu: add utmpname()

This adds `utmpname(3)` on Linux with GNU libc.

Ref: https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/baselib-utmpname-3.html