patch 8.2.1588: cannot read back the prompt of a prompt buffer

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes #6851)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 91fbc67..ac75a7d 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -806,6 +806,7 @@
     {"pow",		2, 2, FEARG_1,	  ret_float,	FLOAT_FUNC(f_pow)},
     {"prevnonblank",	1, 1, FEARG_1,	  ret_number,	f_prevnonblank},
     {"printf",		1, 19, FEARG_2,	  ret_string,	f_printf},
+    {"prompt_getprompt", 1, 1, FEARG_1,	  ret_string,	JOB_FUNC(f_prompt_getprompt)},
     {"prompt_setcallback", 2, 2, FEARG_1, ret_void,	JOB_FUNC(f_prompt_setcallback)},
     {"prompt_setinterrupt", 2, 2, FEARG_1,ret_void,	JOB_FUNC(f_prompt_setinterrupt)},
     {"prompt_setprompt", 2, 2, FEARG_1,	  ret_void,	JOB_FUNC(f_prompt_setprompt)},