MFC r281882(by trasz@):

Simplify linux_getcwd(), removing code that was longer used.

MFC r281829 (by trasz@):

Modify kern___getcwd() to take max pathlen limit as an additional
argument. This will be used for the Linux emulation layer - for Linux,
PATH_MAX is 4096 and not 1024.

MFC r283427:

Where possible we will use M_LINUX malloc(9) type.
Move M_FUTEX defines to the linux_common.ko.

MFC r276564, r276654:

Cast *path to silence clang -Wpointer-sign warning.

Indeed, instead of hiding the kern___getcwd() bug by bogus cast
in r276564, change path type to char * (pathnames are always char *).
And remove bogus casts of malloc().
kern___getcwd() internally doesn't actually use or support u_char *
paths, except to copy them to a normal char * path.

These changes are not visible to libc as libc/gen/getcwd.c misdeclares
__getcwd() as taking a plain char * path.

While here remove _SYS_SYSPROTO_H_ for __getcwd() syscall as
we always have sysproto.h.

MFC r276550:
Remove Giant from linux_getcwd() due to VFS is MPSAFE now.

MFC r276564, r276654:

Cast *path to silence clang -Wpointer-sign warning.

Indeed, instead of hiding the kern___getcwd() bug by bogus cast
in r276564, change path type to char * (pathnames are always char *).
And remove bogus casts of malloc().
kern___getcwd() internally doesn't actually use or support u_char *
paths, except to copy them to a normal char * path.

These changes are not visible to libc as libc/gen/getcwd.c misdeclares
__getcwd() as taking a plain char * path.

While here remove _SYS_SYSPROTO_H_ for __getcwd() syscall as
we always have sysproto.h.

MFC r276550:
Remove Giant from linux_getcwd() due to VFS is MPSAFE now.