menuconfig: Add Save/Load buttons

If menuconfig have Save/Load button like alternative
.config editors, xconfig, nconfig, etc.We will have
a obvious benefit when use menuconfig just like
when we use others, we can Save/Load our .config quickly
and conveniently.

This patch add the Save/Load button for menuconfig.

[remove trailing space while at it for below line:
"*)  Formerly when I used Page Down and Page Up, the cursor would be set"
]

Changes:
V1-V2:
1:use PATH_MAX instead of hard code suggested by Yann E. MORIN
2:drop the spurious empty-line removal suggested by Yann E. MORIN
V2-V3:
1:ajust buttons position well centered reported by Yann E. MORIN

Signed-off-by: Wang YanQing <udknight@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>