provider: Add support for clipboard registers.

This reimplements the '+'/'*' clipboard registers(both are aliases to the same
register, no dedicated storage for the X11 selection) on top of the provider
infrastructure.

This adds two new 'unnamedclip' option, has the same effect of setting
'clipboard' to 'unnamed/unnamedplus' in vim

The 'clipboard' option was not reused because all values(except 'unnamedplus')
seem to be useless for Neovim, and the code to parse the option was relatively
big. The option remains for vim compatibility but it's silently ignored.

src/nvim/option_defs.h

@@ -586,6 +586,7 @@ static char *(p_ttym_values[]) =
 EXTERN char_u   *p_udir;        /* 'undodir' */
 EXTERN long p_ul;               /* 'undolevels' */
 EXTERN long p_ur;               /* 'undoreload' */
+EXTERN int p_unc;               /* 'unnamedclip' */
 EXTERN long p_uc;               /* 'updatecount' */
 EXTERN long p_ut;               /* 'updatetime' */
 EXTERN char_u   *p_fcs;         /* 'fillchar' */
@@ -631,6 +632,7 @@ EXTERN int p_wa;                /* 'writeany' */
 EXTERN int p_wb;                /* 'writebackup' */
 EXTERN long p_wd;               /* 'writedelay' */
 EXTERN char *p_ipy;             // 'initpython'
+EXTERN char *p_icpb;            // 'initclipboard'
 
 /*
  * "indir" values for buffer-local opions.