Global: add new unit related options

1. `--<module>-space-before-unit`
2. `--duration-abbreviation`
3. various code refactoring and cleanups

src/common/frequency.c

@@ -0,0 +1,17 @@
+#include "frequency.h"
+
+bool ffFreqAppendNum(uint32_t mhz, FFstrbuf* result)
+{
+    if (mhz == 0)
+        return false;
+
+    const FFOptionsDisplay* options = &instance.config.display;
+    const char* space = options->freqSpaceBeforeUnit == FF_SPACE_BEFORE_UNIT_NEVER ? "" : " ";
+    int8_t ndigits = options->freqNdigits;
+
+    if (ndigits >= 0)
+        ffStrbufAppendF(result, "%.*f%sGHz", ndigits, mhz / 1000., space);
+    else
+        ffStrbufAppendF(result, "%u%sMHz", (unsigned) mhz, space);
+    return true;
+}