Renamed flags and neededFlags fields in Formatter.

Renamed Formatter's adjust() function to updateFontStyle().  Renamed the
flags and neededFlags fields in the Formatter struct to newFontStyle and
oldFontStyle respectively, and changed their type from int to unsigned.

Renamed macros BOLDFLAG and ULINEFLAG to FONT_STYLE_BOLD and
FONT_STYLE_UNDERLINE respectively.

Added static helper functions hasFontStyle() and didFontStyleChange().