setjmp(2): update path to u.h
 [rsc] --rw-rw-r-- M 487924 glenda sys 1863 Apr  7 11:47 sys/man/2/setjmp
	/n/sourcesdump/2006/0407/plan9/sys/man/2/setjmp:55,63 - 
	/n/sourcesdump/2006/0408/plan9/sys/man/2/setjmp:55,63
	  and
	  .I longjmp
	  can also be used to switch stacks.
	- Defined in
	- .B </$objtype/u.h>
	- are several macros that can be used to build
	+ Several macros are defined in
	+ .B /$objtype/include/u.h
	+ that can be used to build
	  .B jmp_bufs
	  by hand.  The following code establishes a
	  .B jmp_buf