Clean up some code duplication in the header files

PR htop-dev/htop#70 got rid of the infrastructure for generating header
files, but it left behind some code duplication.

Some of cases are things that belong in the header file and don't need
to be repeated in the C file. Other cases are things that belong in the
C file and don't need to be in the header file.

In this commit I tried to fix all of these that I could find. When given
a choice I preferred keeping things out of the header file, unless they
were being used by someone else.

StringUtils.c

@@ -14,12 +14,6 @@ in the source distribution for its full text.
 #include <strings.h>
 #include <stdlib.h>
 
-
-/*
- * String_startsWith gives better performance if strlen(match) can be computed
- * at compile time (e.g. when they are immutable string literals). :)
- */
-
 char* String_cat(const char* s1, const char* s2) {
    int l1 = strlen(s1);
    int l2 = strlen(s2);