CLI: make --mirror available in standard non-batch mode via

new routines `mirror_start()`, `mirror_outfile()` & limit
  substitutions on Unix to backslash (Windows remains the same);
  restrict "borderwidth", "textgap", "vwhitesp", "whitesp" < 100
CODEONE: fix `ceilf()` -> `C1_MULT_CEIL()` in FAST_MODE encodation
  (improves/changes some encodation)
composite: preserve `gs1_verify()` warning using `warn_level` hack;
  fix wrong name `zint_dbar_omnstk_set_height()` ->
  `zint_dbar_stk_set_height()`;
  use new `zint_micropdf_variant()` & `zint_micropdf_expand()`
  routines (see below)
MICROPDF417: move variant determination into new shareable routine
  `zint_micropdf_variant()` & expansion into `zint_micropdf_expand()`
qzint: add helper methods `setbordertypevalue()`,
  `encodedinputmode()`, `encodedoutputoptions()`,
  `ecivaluetoeciindex()` and `eciindextoecivalue()`
dbar: for performance pre-calculate `combins()` as array
coverage: add `malloc()`-type failure testing to AZTEC,
  CONTENT_SEGS, MEMORY_FILE, raster, Reed-Solomon & vector;
  numerous changes to remove unused code and test more branches
common: add `isxdigit()`
general: various code fiddlings
manual: add char names to sequence and batch format char tables;
  mention long options can be shortened (& use "--compliantheight"
  -> "--compliant");
  expand mirror explanation & document substitutions in footnote;
  workaround xecjk putting space after double quote when quoting
  cjk chars & rejig input mode examples;
  workaround html alignment bug in table gs1-enabled symbologies by
  adding extra rh dashes to header cols;
  add "figure nn: " to txt image tags via sed:
  various other small fixes and rephrasings

backend/filemem.h

@@ -90,6 +90,25 @@ INTERNAL int zint_fm_error(struct filemem *restrict const fmp);
    NOTE: don't use, included only for libpng compatibility */
 INTERNAL int zint_fm_flush(struct filemem *restrict const fmp);
 
+#ifdef ZINT_TEST
+/* For testing `malloc()`/`realloc()` failures */
+#define FM_FAIL_ID_OPEN     1
+#define FM_FAIL_ID_WRITE    2
+#define FM_FAIL_ID_PUTC     3
+#define FM_FAIL_ID_PUTS     4
+#define FM_FAIL_ID_PRINTF   5
+#define FM_FAIL_ID_PUTSF    6
+#define FM_FAIL_ID_CLOSE    7
+#define FM_FAIL_ID_SEEK     8
+#define FM_FAIL_ID_TELL     9
+#define FM_FAIL_ID_FLUSH    10
+#define FM_FAIL_ID_MALLOC   11
+#define FM_FAIL_ID_REALLOC  12
+
+INTERNAL void zint_test_fm_set_fail(const int id, const int at);
+INTERNAL void zint_test_fm_set_alloc_fail(const int id, const int at);
+#endif
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */