libsysdecode: decode PROT_MAX flags

Extend libsysdecode to pretty-print PROT_MAX flags and fix decoding of regular protection flags broken since r349240.

before:
truss:
mmap(0x0,40960,0x30000,MAP_PRIVATE|MAP_ANON|MAP_NOCORE,-1,0x0) = 34366234624 (0x800632000)
kdump:
11439 protmax CALL mmap(0,0xa000,0x30000<><invalid>196608,0x21002<MAP_PRIVATE|MAP_ANON|MAP_NOCORE>,0xffffffff,0)

after:
truss:
mmap(0x0,40960,PROT_MAX(PROT_READ|PROT_WRITE)|PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANON|MAP_NOCORE,-1,0x0) = 34366234624 (0x800632000)
kdump:
11439 protmax CALL mmap(0,0xa000,0x30000<PROT_MAX(PROT_READ|PROT_WRITE)|PROT_READ|PROT_WRITE>,0x21002<MAP_PRIVATE|MAP_ANON|MAP_NOCORE>,0xffffffff,0)

Reviewed by: kib (mentor)
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D21706