Allow -u UID

Defaults to username, of course, like all POSIX utilities do

CommandLine.c

@@ -11,6 +11,7 @@ in the source distribution for its full text.
 #include "CommandLine.h"
 
 #include <assert.h>
+#include <ctype.h>
 #include <getopt.h>
 #include <locale.h>
 #include <stdbool.h>
@@ -178,8 +179,12 @@ static CommandLineSettings parseArguments(const char* program, int argc, char**
             if (!username) {
                flags.userId = geteuid();
             } else if (!Action_setUserOnly(username, &(flags.userId))) {
-               fprintf(stderr, "Error: invalid user \"%s\".\n", username);
-               exit(1);
+               for (const char *itr = username; *itr; ++itr)
+                  if (!isdigit((unsigned char)*itr)) {
+                     fprintf(stderr, "Error: invalid user \"%s\".\n", username);
+                     exit(1);
+                  }
+               flags.userId = atol(username);
             }
             break;
          }

htop.1.in

@@ -62,7 +62,7 @@ Show only the given PIDs
 Sort by this column (use \-\-sort\-key help for a column list).
 This will force a list view unless you specify -t at the same time.
 .TP
-\fB\-u \-\-user=USERNAME\fR
+\fB\-u \-\-user=USERNAME|UID\fR
 Show only the processes of a given user
 .TP
 \fB\-U \-\-no-unicode\fR