Update mq_open(2) to document an error case and mqueuefs(5) to document sysctl variables

[felix.the.red_gmail.com]

Summary

This documents a case where mq_open can fail and return EINVAL (see PR 243209)
if doing so would exceed limits specified by sysctl values.
Also, document the mqueue sysctls.

[markj]

lib/libc/sys/mq_open.2

281

I think this is ok but perhaps deserves some additional qualification: the limits are only relevant if O_CREAT is set, and maxmsgsize and maxmsg are only checked if the mq_open(O_CREAT) call also specifies a mq_attr.

314

mqueuefs(5) might be a better place to document these?

[felix.the.red_gmail.com]

Move sysctl documentation to mqueuefs(5).
Qualify description for EINVAL error to note the use of O_CREAT and attr.

[markj]

share/man/man5/mqueuefs.5

126

I am a bit skeptical that it is worth documenting default values: if they ever change, this document will be out of date, and users can find the default values simply by reading the sysctl.