iconv.1, pthread_rwlockattr_setkind_np.3, man-pages.7, socket.7, iconvconfig.8: Standardize on "nonzero"

Also add this term to the style guide in man-pages(7).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man1/iconv.1

@@ -112,7 +112,7 @@ Print a short usage summary and exit.
 Print the version number, license, and disclaimer of warranty for
 .BR iconv .
 .SH EXIT STATUS
-Zero on success, non-zero on errors.
+Zero on success, nonzero on errors.
 .SH ENVIRONMENT
 Internally, the
 .B iconv

man3/pthread_rwlockattr_setkind_np.3

@@ -100,7 +100,7 @@ Given valid pointer arguments,
 always succeeds.
 On error,
 .BR pthread_rwlockattr_setkind_np ()
-returns a non-zero error number.
+returns a nonzero error number.
 .SH ERRORS
 .TP
 .BR EINVAL

man7/man-pages.7

@@ -682,6 +682,7 @@ filesystem	file system
 hostname	host name
 inode	i-node
 lowercase	lower case, lower-case
+nonzero	non-zero
 pathname	path name
 pseudoterminal	pseudo-terminal
 privileged port	T{

man7/socket.7

@@ -319,7 +319,7 @@ or an extended BPF
 program to the socket for use as a filter of incoming packets.
 A packet will be dropped if the filter program returns zero.
 If the filter program returns a
-non-zero value which is less than the packet's data length,
+nonzero value which is less than the packet's data length,
 the packet will be truncated to the length returned.
 If the value returned by the filter is greater than or equal to the
 packet's data length, the packet is allowed to proceed unmodified.

man8/iconvconfig.8

@@ -87,7 +87,7 @@ Print a short usage summary and exit.
 Print the version number, license, and disclaimer of warranty for
 .BR iconv .
 .SH EXIT STATUS
-Zero on success, non-zero on errors.
+Zero on success, nonzero on errors.
 .SH FILES
 .TP
 .I /usr/lib/gconv