Correct the regressive part of my last commit to these files:

use the .Fn macro instead of the .Fo ... .Fc combination to
format function prototypes.

Reminded by:	bde

lib/libc/sys/sigaction.2

@@ -57,10 +57,8 @@ struct  sigaction {
 #define	sa_sigaction	__sigaction_u.__sa_sigaction
 .Ed
 .Ft int
-.Fo sigaction
-.Fa "int sig" "const struct sigaction * restrict act"
-.Fa "struct sigaction * restrict oact"
-.Fc
+.Fn "int sig" "const struct sigaction * restrict act" \
+    "struct sigaction * restrict oact"
 .Sh DESCRIPTION
 The system defines a set of signals that may be delivered to a process.
 Signal delivery resembles the occurrence of a hardware interrupt:

lib/libc/sys/sigprocmask.2

@@ -43,9 +43,8 @@
 .Sh SYNOPSIS
 .In signal.h
 .Ft int
-.Fo sigprocmask
-.Fa "int how" "const sigset_t * restrict set" "sigset_t * restrict oset"
-.Fc
+.Fn sigprocmask "int how" "const sigset_t * restrict set" \
+    "sigset_t * restrict oset"
 .Sh DESCRIPTION
 The
 .Fn sigprocmask