backlight: accept '%' in the brightness input value

Improve friendlyness of the command line by accepting the percent brightness
in both format: with or without a trailing '%'

Reviewed by:	manu
Differential Revision:	https://reviews.freebsd.org/D26692

usr.bin/backlight/backlight.8

@@ -22,7 +22,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 02, 2020
+.Dd October 06, 2020
 .Dt BACKLIGHT 8
 .Os
 .Sh NAME
@@ -63,6 +63,7 @@ When querying the brightness level only print the value.
 Query information about the backlight (name, type).
 .It Ar value
 Set the brightness level to this value, must be between 0 and 100.
+A trailing '%' is valid.
 .It Ar incr | +
 .Op Ar value
 Decrement the backlight level.

usr.bin/backlight/backlight.c

@@ -144,6 +144,9 @@ main(int argc, char *argv[])
 			action = BACKLIGHT_SET_BRIGHTNESS;
 
 		if (argc == 1) {
+			/* ignore a trailing % for user friendlyness */
+			if (argv[0][strlen(argv[0]) - 1] == '%')
+				argv[0][strlen(argv[0]) - 1] = '\0';
 			percent = strtonum(argv[0], 0, 100, &percent_error);
 			if (percent_error)
 				errx(1, "Cannot parse brightness level %s: %s",