mntgen: import mntgen(4) from Plan 9

new file:   man/man4/mntgen.4
new file:   src/cmd/mntgen.c

man/man4/mntgen.4

@@ -0,0 +1,32 @@
+.TH MNTGEN 4
+.SH NAME
+mntgen  \-  automatically generate mount points for file systems
+.SH SYNOPSIS
+.B mntgen
+[
+.B -s
+.I service
+]
+[
+.I mnt
+]
+.SH DESCRIPTION
+.I Mntgen
+mounts itself on
+.I mnt
+(default
+.BR /n )
+after the current contents,
+creating subdirectories on demand as they are accessed.
+It is intended to supply mount points automatically.
+.PP
+The
+.B -s
+option causes
+.I mntgen
+to post a 9P
+.I service \fR
+file.
+.SH SOURCE
+.B \*9/src/cmd/mntgen.c
+