Correct and/or/div/mod in path expressions

The grammar is creating implicit lexer tokens for and/or/div/mod, which
take precedence overt NCName. Make sure we define these tokens
explicitly and allow them to be an alternative wherever NCName is
valid.

JIRA: YANGTOOLS-1387
Change-Id: I4f4f26fe0f8bc27614b769b465a76696a49ad7c2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 1a7e93a50d22944771fe2ea4fc18c24fcf3aae11)