MFC r305093 (by mjg@):

fd: add fdeget_locked and use in kern_descrip

MFC r305756 (by oshogbo@):

fd: add fget_cap and fget_cap_locked primitives.
They can be used to obtain capabilities along with a referenced fp.

MFC r306174 (by oshogbo@):

capsicum: propagate rights on accept(2)

Descriptor returned by accept(2) should inherits capabilities rights from
the listening socket.

PR: 201052

MFC r306184 (by oshogbo@):

fd: simplify fgetvp_rights by using fget_cap_locked.

MFC r306225 (by mjg@):

fd: fix up fgetvp_rights after r306184

fget_cap_locked returns a referenced file, but the fgetvp_rights does
not need it. Instead, due to the filedesc lock being held, it can
ref the vnode after the file was looked up.

Fix up fget_cap_locked to be consistent with other _locked helpers and not
ref the file.

This plugs a leak introduced in r306184.

MFC r306232 (by oshogbo@):

fd: fix up fget_cap

If the kernel is not compiled with the CAPABILITIES kernel options
fget_unlocked doesn't return the sequence number so fd_modify will
always report modification, in that case we got infinity loop.

MFC r311474 (by glebius@):

Use getsock_cap() instead of fgetsock().

MFC r312079 (by glebius@):

Use getsock_cap() instead of deprecated fgetsock().

MFC r312081 (by glebius@):

Use getsock_cap() instead of deprecated fgetsock().

MFC r312087 (by glebius@):

Remove deprecated fgetsock() and fputsock().

Bump __FreeBSD_version as getsock_cap changed and
fgetsock/fputsock pair removed.