allow specifying larger /tmp tmpfs mounts

Extract the size string-parsing and overflow-checking wrapper into util.c.

Test: New unit tests.
Change-Id: I31ba2f1a77217a2f13cda078e5e6a80104fbcd32
Signed-off-by: Martin Pelikán <[email protected]>
diff --git a/util.h b/util.h
index f019f00..f835f17 100644
--- a/util.h
+++ b/util.h
@@ -58,6 +58,7 @@
 const char *lookup_syscall_name(int nr);
 
 long int parse_constant(char *constant_str, char **endptr);
+int parse_size(size_t *size, const char *sizespec);
 
 char *strip(char *s);
 char *tokenize(char **stringp, const char *delim);