Reformat syscalls.master for better readability.

This takes advantage of two recents changes to makesyscalls.sh:
r328598: Permit a range of syscall numbers for UNIMPL
r339624: Remove the need for backslashes in syscalls.master

Syscall declerations are now split across multiple lines with the
syscall name and variables each on seperate lines (with an exception for
syscalls taking no arguments.)

Reviewed by: imp
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17706