Optimize Strings_startWith()

Use strncmp() combined with a strlen() will give better performance
than a strstr in worst case. Especially when the match prefix is a
constant and not a variable.

While we are at it, replace the match() function in linux/Battery.c,
which uses a naive algorithm, with a macro that does better job by
utilizing Strings_startWith().

    $ gcc --version | head -n 1
    gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
    $ uname -m
    x86_64
    $ size htop.old htop.new
       text   data    bss    dec    hex filename
     137929  15112   3776 156817  26491 htop.old
     137784  15104   3776 156664  263f8 htop.new

Signed-off-by: Kang-Che Sung <explorer09 @ gmail.com>

StringUtils.c

@@ -17,10 +17,15 @@ in the source distribution for its full text.
 /*{
 #include <stdio.h>
 
-#define String_startsWith(s, match) (strstr((s), (match)) == (s))
+#define String_startsWith(s, match) (strncmp((s),(match),strlen(match)) == 0)
 #define String_contains_i(s1, s2) (strcasestr(s1, s2) != NULL)
 }*/
 
+/*
+ * 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);