libcasper: fix mandoc lint warnings in services man pages

[xiujunma]

What

Fix three real mandoc -T lint -W warning warnings in lib/libcasper/services/ man pages.

Changes

• cap_net.3 — drop trailing comma on the last .Nm before .Nd (mdoc convention).
• cap_netdb.3 — same: drop trailing comma on the single .Nm before .Nd.
• cap_pwd.3 — change .Xr cap_endpwent 3 to .Xr endpwent 3. The page documents cap_endpwent, so .Xr cap_endpwent 3 is a self-reference. The surrounding text describes the cap_pwd functions as "respectively equivalent to" a list of libc functions (getpwent, getpwnam, setpwent, …) — the intended trailing entry is therefore endpwent(3). This is the only content change in the PR.

Verification

mandoc -T lint -W warning before:

mandoc: lib/libcasper/services/cap_net/cap_net.3:43:36: WARNING: bad NAME section content: text
mandoc: lib/libcasper/services/cap_net/cap_net.3:46:5: WARNING: unknown library name: Lb libcap_net
mandoc: lib/libcasper/services/cap_netdb/cap_netdb.3:28:24: WARNING: bad NAME section content: text
mandoc: lib/libcasper/services/cap_netdb/cap_netdb.3:31:5: WARNING: unknown library name: Lb libcap_netdb
mandoc: lib/libcasper/services/cap_pwd/cap_pwd.3:43:5: WARNING: unknown library name: Lb libcap_pwd
mandoc: lib/libcasper/services/cap_pwd/cap_pwd.3:93:5: WARNING: cross reference to self: Xr cap_endpwent 3

mandoc -T lint -W warning after:

mandoc: lib/libcasper/services/cap_net/cap_net.3:46:5: WARNING: unknown library name: Lb libcap_net
mandoc: lib/libcasper/services/cap_netdb/cap_netdb.3:31:5: WARNING: unknown library name: Lb libcap_netdb
mandoc: lib/libcasper/services/cap_pwd/cap_pwd.3:43:5: WARNING: unknown library name: Lb libcap_pwd

The three remaining unknown library name: Lb libcap_* warnings are intentionally out of scope. The .Lb libcap_* invocations in the pages are correct mdoc; the warnings come from mandoc's bundled library-name database not knowing these names. Fixing that belongs in mandoc upstream, not in these pages.

Out of scope

• cap_sysctl.3 has a sections out of conventional order: Sh RETURN VALUES warning. Real defect; saved for a separate, larger PR because the fix requires moving a whole section.
• The Lb libcap_* warnings noted above.