mach: Fix incoherency between perror and strerror

08d2024b ("string: Simplify strerror_r") inadvertently made
__strerror_r print unknown error system in decimal while the original
code was printing it in hexadecimal. perror was kept printing in
hexadecimal in 725eeb4a ("string: Use tls-internal on strerror_l"),
let us keep both coherent.

This also fixes a duplicate ':'

Spotted by the libunistring testsuite test-perror2