std.manifestIni should support repeated section names

Hello. This is a change request similar in spirit to https://github.com/google/jsonnet/issues/386 (fixed in https://github.com/google/jsonnet/commit/d4434c8df41c46d4ed11958677a655f0d1c52f87), but this time to allow sections with duplicate names (instead of duplicate keys).

The INI file wiki acknowledges this exists. As a specific example, wireguard needs multiple `[Peer]` sections.

Modifying the upstream `std` like this, by adding the array-check in `section_lines`, seems to work (now if a value of a key within `sections` is an array, then that section will be repeated):
```
manifestIni(ini):
local body_lines(body) =
std.join([], [
local value_or_values = body[k];
if std.isArray(value_or_values) then
['%s = %s' % [k, value] for value in value_or_values]
else
['%s = %s' % [k, value_or_values]]

for k in std.objectFields(body)
]);

local section_lines(sname, value_or_values) =
if std.isArray(value_or_values) then
local rep = value_or_values;
std.join([], [
['[%s]' % [sname]] + body_lines(sbody)
for sbody in rep
])
else
local sbody = value_or_values;
['[%s]' % [sname]] + body_lines(sbody),
main_body = if std.objectHas(ini, 'main') then body_lines(ini.main) else [],
all_sections = [
section_lines(k, ini.sections[k])
for k in std.objectFields(ini.sections)
];
std.join('\n', main_body + std.flattenArrays(all_sections) + ['']),
```