do not expect mandoc(1) to terminate in finite time

PR:		266868

usr.bin/man/man.sh

@@ -28,6 +28,12 @@
 #
 # $FreeBSD$
 
+# Rendering a manual page is fast. Even a manual page several 100k in size
+# takes less than a CPU second. If it takes much longer, it is very likely
+# that a tool like mandoc(1) is running in an infinite loop. In this case
+# it is better to terminate it.
+ulimit -t 20
+
 # Usage: add_to_manpath path
 # Adds a variable to manpath while ensuring we don't have duplicates.
 # Returns true if we were able to add something. False otherwise.