Don't concatenate multiple JS scripts in jerry

Jerry (the command line tool) has been supporting the execution of
multiple script files for long. However, until now, it simply
concatenated all sources into a single source buffer and
parsed/executed them as one unit. Other JS execution tools (e.g.,
jsc, v8) load and execute separate files as separate units -- but
still in the same execution environment. The most significant
effect of this approach is that the `"use strict;"` directive (or
the absence of it) at the beginning of each JS script file takes
effect as expected (i.e., as if the script was executed alone).
Contrarily, the concatenation-based approach forces the strictness
of the first script on all the rest (i.e., if the first script
starts with `"use strict";` the rest is also executed in a strict
environment even if they did not contain the directive, and vice
versa).

This patch makes the jerry command line tool to load/parse/run one
unit at a time.

Side effects:
- As there is no need for separate file read routines that load one
  file (a snapshot) or concat multiple (JS sources) anymore, those
  routines got merged.
- Both previous read routines used multiple stdio functions
  (`fseek`, `ftell`, and `rewind`). This has been simplified to
  rely on `fread` only to find out the length of the input.
- This simplification made the above mentioned functions
  superfluous in jerry-libc.
- As some error messages had to be touched in this patch, several
  more have been beautified to make them more consistent.
- One small change was needed in `jerry_parse` in jerry-core to
  allow subsequent parsing of multiple sources (without that, an
  assertion was triggered).

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-libc/include/stdio.h

@@ -37,13 +37,6 @@ extern FILE *stdin;
 extern FILE *stdout;
 extern FILE *stderr;
 
-/**
- * fseek's 'whence' argument values
- */
-#define SEEK_SET 0
-#define SEEK_CUR 1
-#define SEEK_END 2
-
 /**
  * I/O routines
  */
@@ -52,10 +45,7 @@ FILE *fopen (const char *path, const char *mode);
 int fclose (FILE *fp);
 size_t fread (void *ptr, size_t size, size_t nmemb, FILE *stream);
 size_t fwrite (const void *ptr, size_t size, size_t nmemb, FILE *stream);
-int fseek (FILE *stream, long offset, int whence);
-long ftell (FILE *stream);
 int printf (const char *format, ...);
-void rewind (FILE *stream);
 int fprintf (FILE *stream, const char *format, ...);
 
 #ifdef __cplusplus